I'd have to agree rush being slightly under-represented in the environment currently, but that on the Joust side. In Melee there are several very dominant decktypes for Rush (Kennon's GJ from LCG Days being a good example, über-Viper's are another one), which are very competitive. The difficulty in giving, for example, Baratheon a huge boost in rush is to find a balance in boosting them in Joust, while not making them too OP in Melee. And that, if anything, is the reasoning I see behind errata'ing Heir, because that's where it would have really gotten ridiculous. Of course one solution would be to have some "if you have only one opponent" stuff added, since it would only affect the Joust side.
That said, I think Twn2dn and Stag are pretty much right, what the game needs is a real glass cannon. Something that can be broken by applying adequate force, but makes the opponent have to scramble on for dear life from the get-go. Even if they are playing Martell. Instead, the only rush-related cards Bara has been getting after the King's Landing Cycle (which had KoF, arguably one of the best glass-cannon cards Bara has gotten) in the CPs are support- and resilience type cards, like the new Mr. Lomys. Okay, there's one good power-rush Melissandre, but that's about it. Cards that actually give you the possibility to get power faster have been few and far between. Of course, one of the primary reasons for this has been the development of two completely new strands into the Baratheon cacophony... Asshai (mainly control-ish, even though Melissandre + Power of Faith = Good Rush) and Knights (mainly beef and aggro, quite little actual rush anywhere, at least in the newer cards). This current set of CPs seems to be slightly better, with Stormlands Bastard (Renown Queen of Thorns with Kingdom of Shadows!) and Dragonstone Watchtower (I can see myself naming BotS quite often) both being cards that have at least some potential. Now if we only got an event that gave all Bastards renown for a turn...
But I really do think that the game is slightly missing the feel of 'action' that proper glass-cannon rush builds would bring. And not just those "my lords cannot be killed, and they go up and down and up and down..." things, which really can't be speeded up too much anymore, since they're a bit too resilient already by now. Something that would make playing GTM on turn 1 feel risky, since that means having 2 less gold to play characters etc.
Of the other Houses with any Rush potential only Targaryen seems to be developing it further, while Stark, Martell and Greyjoy all seem to be moving AWAY from power-grab instead of in that direction...